A strong, isolated pole gives rise to periodic structure in backward-angle excitation functions. A moderate pole interfering with a nonresonant diffraction amplitude produces similar structure. The ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ + ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ backward-angle elastic excitation function is analyzed from these points of view.

• Received 29 December 1978

DOI:https://doi.org/10.1103/PhysRevLett.42.633